The purpose of this study is to compare the efficacy of therapy with ZDV versus ddI versus combination therapy of ZDV plus ddI in symptomatic HIV-infected children as determined by survival and disease progression (based on growth failure, changes in neuropsychological status or development of > 2 new or recurrent opportunitistic infections) prior to crossover. Additional secondary objectives will be to compare the relative safety and tolerance of these doses based on clinical and laboratory assessments.